The antiferroelectric liquid crystal was discovered by Chandani et al. in 1988 (Chandani et al., Jpn.J.Appl.Phys., 27, L279 (1988)), and has been proposed as a future material substituting for nematic liquid crystals which are presently used in liquid crystal displays.
The switching time of the nematic liquid crystal which is presently used in liquid crystal displays is typically as slow as 30 milliseconds. Thus, these display devices are fabricated using a driving method employing a thin film transistor (TFT) called an active matrix having a very high production cost. Furthermore, a display based on a twisted nematic (TN) system is disadvantageous in that it essentially provides a narrow viewing angle.
The antiferroelectric liquid crystal has a switching time as fast as several tens of microseconds. In the antiferroelectric liquid crystal, liquid crystal molecules respond in plane such that a wide viewing angle can be obtained. Furthermore, the antiferroelectric liquid crystal exhibits a definite threshold voltage even under an applied DC voltage and is thus easily driven. Accordingly, the antiferroelectric liquid crystal can constitute a display device driven by a simple matrix requiring a low production cost.
On the other hand, a ferroelectric liquid crystal disclosed by Meyer et al. in 1975 (R. B. Meyer et al., J. Phys. (France), 36, L69, (1975)) was proposed as a high definition liquid crystal display because of its fast response. However, a full color image display using the ferroelectric liquid crystal has not yet been realized because of the difficulty in producing gray levels. With the antiferroelectric liquid crystal, a gray scale has been realized. Therefore, a full-color animation display has already been achieved, though on an experimental basis.
As mentioned above, the liquid crystal display employing an antiferroelectric liquid crystal is expected to achieve a high definition display having a wide viewing angle using a low cost simple matrix.
However, compounds exhibiting an antiferroelectric liquid crystal phase are extremely restricted from a structural viewpoint. Most of these compounds have a terminal structure such as a 1-substituted alkyl benzoic acid ester exemplified by the following structural formula (see Future Liquid Crystal Display and Its Materials, supervised by Atsuo Fukuda, CMC, (1992)): ##STR1## wherein Y represents an alkyl group or a perfluoroalkyl group.
The antiferroelectric liquid crystal incorporated into display devices must have a practical working temperature range such as nematic liquid crystals in general use and must also exhibit various properties relating to display quality. Thus, it is necessary to mix antiferroelectric liquid crystal compounds having different properties to form the desired liquid crystal composition. However, because the structure that allows the appearance of an antiferroelectric liquid crystal phase is restricted as mentioned above, the only allowable structural modifications are the introduction of an alicyclic group, condensed ring or heterocyclic group into the core structure and the introduction of an ether bond into the chiral terminal alkyl chain. The resulting compound is much like the original compound in its properties. Thus, a practical liquid crystal composition having the above properties is difficult to obtain.
Thus, the development of liquid crystal compounds having a range of properties is indispensable in obtaining an antiferroelectric liquid crystal having practically useful properties.
On the other hand, the present inventors have found that a compound having the following structure can exhibit an antiferroelectric liquid crystal phase (see JP-A-4-82862 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). ##STR2##
This compound has 2-methylalkanoic acid incorporated therein and thus exhibits a stable antiferroelectric liquid crystal phase. However, this compound has a high viscosity because its core structure is similar to that of known antiferroelectric liquid crystals and also has a high threshold voltage. The present inventors also found that an antiferroelectric liquid crystal phase appears when 2-methylalkanoic acid is introduced into the core of a heterocyclic compound such as diphenylpyrimidine (see JP-A-9-31063). This compound has a lower viscosity than the foregoing ester compounds and thus is useful. However, this compound also has a high threshold voltage. That is, there is a need to solve the above problems of the prior art.